In the field of oil and gas exploration and extraction, pressure sensors are customarily used at the surface for reading data provided by acoustic transducers at the downhole. The data travels through the drilling mud along the wellbore, typically in the form of short pulses providing a binary encoded signal. One of the most severe interference sources for mud pulse telemetry is the perturbation generated by the pumps that circulate the mud. Many attempts have been made to reduce or eliminate pump interference. For example, some attempts include the use of two or more sensors having a well-known signal delay between one another. Other approaches include averaging algorithms combined with pump stroke monitors to generate a signature of the pump interference. Some of these methods rely on assumptions such as the shape of the pump interference being the same or similar for different sensors. In other methods the outputs of the two sensors are used to calculate the transfer function between the sensors and from that, the received signal. However, these approaches are hindered by the small difference typically encountered between the signals of the two or more sensors, even when they are placed far apart from each other, as compared to the amplitude of pump interference.
In many instances circulation and drilling must be stopped in order to collect reference data and elaborate complex mathematical models are needed for interference rejection. Some of the mathematical models used include cancellation of the harmonics of pump interference using Fast Fourier Transform (FFT) to generate a reference signal representing pump cycles. Calculations that are more sophisticated include interpolation of out-of-band frequency components of the pump interference to find in-band harmonics and generate a reference signal. Some approaches use linear prediction to generate an all-pole model of the pump interference, where a delayed version of a received signal is used to estimate pump interference. In further approaches, a known sequence of pulses is transmitted at least twice through the system (in both directions) to accurately calculate a transfer function between the deployed sensors.
Most systems use large ‘acoustic’ capacitors to act as pump dampeners. These devices operate as large balloons made of a resilient material that swells with drilling mud, thus acoustically isolating a pressure sensor from the pumps. Still, ‘acoustic’ capacitors are unable to provide the level of attenuation desired when an acoustic transducer is far deep inside a wellbore. More generally, state-of-the-art modelling of pump interference neglects data transfer noise sources, such as drill bit noise in the wellbore. Furthermore, techniques such as described above are time consuming and expensive in terms of instrumentation, involving a plurality of acoustic transducers and sensitive detection equipment.